You can, but you better get in writing from "their" doctor that you are health and fit for work. At least if you go back out, it was their doctor who said you were fit. If you have a serious back issue, this job isn't worth holding onto to. Especially if you're already hoping you can make it to 180 days just to back out again. So in that case, get a private doctor and have them run tests to see how damaged your disks are. Compressions, pinched nerves or slip disks, you do not want to put this job over any of those injuries. Now if you're just sandbagging, we had a person go out for 89 days with a "Knee", worked for a few weeks and then when back out with an "Ankle". I believe the "180 days" only applies to the same injury. I could be wrong but I know that person didn't stay for the 180 before going back out again.